Absorbable synthetic polymers usable for medical products such as e.g. surgical suture material or implants, include conventional homopolymers of polyglycolic acid or polylactic acid, as well as their copolymers. Particularly in the case of suture materials monofilament products have the advantage compared with braided multifilament structures, that they have a smooth, homogeneous surface. This facilitates the course of the thread and reduces the occurrence of capillarities. Thus, no coatings need be applied in order to improve the compactness of the thread and the pull-through forces through the tissue to be sewn are reduced due to the smooth structure.
A disadvantage of the known polymers for suture material is their high flexural strength, partly linked with an inadequate transverse tensile strength, which leads to a poor knotting behaviour and limits the use for surgical sutures.
Therefore developments have led to the use of block copolymers, e.g. the structure AB, ABA or ABAB, in which at least one block constitutes a so-called soft segment. It is known to produce soft segments by homopolymerization or copolymerization of monomers, such as e.g. trimethylene carbonate (1,3-dioxan-2-one) TMC, .epsilon.-caprolactone or p-dioxanone (1,4-dioxan-2-one). The soft segments are reacted with hard segments, whose monomers are typically chosen from glycolide and/or lactide, to the corresponding block copolymers.
Among the commercially marketed long term-absorbable suture materials reference is made to the block copolymer of glycolide and trimethylene carbonate disclosed in European patent 98394 Al of the American Cyanamid Company. A crystalline copolymer of glycolide and .epsilon.-caprolactone described in European patent EP 441322 A1 of ETHICON Inc. constitutes a short term-absorbable polymer material.
European patent EP 626404 A2 of United States Surgical Corporation (USSC) claims absorbable block copolymers of glycolide, p-dioxanone and trimethylene carbonate, in which the soft segment is formed solely from p-dioxanone and TMC.
U.S. Pat. No. 5,431,679 of United States Surgical Corporation describes a block copolymer, which comprises a block of glycolide ester units and a block of statistical copolymers of 1,3-dioxan-2-one and caprolactone.